In apparatus of the type disclosed in U.S. Pat. No. 3,982,622, granted on Sept. 28, 1976, to J. A. Bellino et al. alphanumeric indicia are printed on paper that is wrapped around a printing platen. The printing is accomplished by selectively printing columns of dots in rows across the paper from a left-hand margin to a right-hand margin. The combinations of columns of selectively printed dots form the shapes of the alphanumeric characters. In controlling such a printing apparatus, it is necessary to keep track of the position of the carriage. It is particularly important to prevent the carriage from proceeding too far to the left or the right so as to exceed the width of the paper or to strike the framework of the printing apparatus.
U.S. Pat. No. 3,923,137, granted on Dec. 2, 1975, to Kashio discloses digital storage for storing the printing position count corresponding to the left margin and the printing position count corresponding to the right margin. A print position counter than keeps track of the location of the printing apparatus. Coincidence circuits are used to indicate when the printing apparatus has reached a margin of the printer. A return switch on the printing apparatus of the Kashio patent detects the return of the print head to the starting position. Similarly, a switch is provided on the printing apparatus for detecting the arrival of the printing head at the rightmost position of the printing field.
U.S. Pat. No. 3,463,081, granted on Aug. 26, 1969, to A. B. Levine, discloses a ring counter having a number of stages equal to the maximum number or columns of symbols to be printed in each line, to keep track of the printing of characters across the width of a page. However, the Levine patent relates to a belt printer.